The research aims to establish the biochemical basis of calcium absorption across the small intestine and the mechanisms of regulation by vitamin D, dietary calcium, growth and aging, pregnancy and other factors. The absorption involves active transport of calcium and comprises at least two identified steps: entry at the mucosal surface and exit via the contraluminal plasma membrane. Vitamin D maintains tissue components of the cation pump and their identification and characterization is the focus of this investigation. A particulate calcium-binding complex dependent on vitamin D has been identified in rat mucosal homogenates. The membrane preparation has been solubilized and a new vitamin D-dependent protein which binds calcium with high affinity isolated. The role of this protein, named IMCal, and of a vitamin D-dependent calcium-dependent adenosine triphosphatase is under investigation.